Wind affects water trap seals, and can cause traps within the building to self-siphon. This usually occurs when wind gusts are over 35 km/h. When the wind blows over the stack outlet, the air pressure in the pipe is lowered. This is due to Bernoulli’s principle: “As the speed of the moving fluid (air) increases, the pressure within the fluid (air) decreases”. The lowered pressure in the pipe creates suction, negative transients, throughout the above-ground drainage system. As the wind gusts, this causes the trap seals typically closest to the open vent to oscillate, and can cause the trap to self-siphon. If the gust causes significant pressure reaching – 400Pa the water traps seals can be depleted.
What is the best way to deal with wind shear at the top of the building influencing the pipe pressures?
If the water trap seals are not protected (and air passes through), smells and disease can enter into apartment units. Since it is the only barrier between the drainage system and the living and/or working space, it is critical that the water trap seal is maintained at all times, to protect the occupants of the building from potentially harmful gases and pathogens from within the drainage system.
For these reasons, it’s crucial that high-rise building design, where winds easily reach well over 35 km/h, should incorporate wind shear protection to avoid any risk of self-siphonage.